The present Invention relates, in general, to a novel window seal capable of transmitting infrared radiation for use under high temperature ultra-high vacuum conditions.
In multiple infrared (IR) photon laser-induced chemistry, the need arises to contain extremely small samples of low-volatility compounds totally in the gas phase at pressures of some millitorr during several hour irradiations with a high power pulsed infrared laser. In practice, the most significant obstacle has been the absence of a non-fragile, large aperture, chemically inert, vacuum window seal capable of long-term operation at temperatures of 200.degree.-275.degree. C. While the ultimate pressures involved are not in the ultra-high vacuum range, sealed-off operation of the cell places equivalent requirements on the leak and outgas rates of the seals. Furthermore, because of the high laser damage thresholds, low indices of refraction, wide spectral passband, and low costs of alkali halides relative to other IR window materials, sealing to ductile-brittle materials such as NaCl, KCl, and KBr is required.
Various methods for sealing infrared windows to metal substrates are known in the art; however, none of these methods are usable under conditions of simultaneous high vacuum and high temperatures. Also, prior art seals are either unable to be temperature cycled or require very long heat-up and cool-down times. For example, O-rings and various elastomer materials all fail between room temperature and approximately 150.degree. C. because of air permeability and sealant decomposition accompanied by continuous outgassing and long-term loss of elasticity. In addition, these materials often show reactivity with various chemical reagents.
Various metallic seals (gold and other overlapped thin wire seals, "K rings") do not have the permeability and outgassing problems of elastomers, but typically fail in the areas of reliability, fragility, difficulty in assembly, and temperature cycling. AgCl seals cannot be used directly on substrates other than silver or gold, are difficult to prepare, and are unreliable in temperature cycled applications.
Silicone resin seals also appear to have problems in temperature cycling in that they require inordinately long times to heat or cool (on the order of weeks) and the seal itself is sensitive to temperature fluctuations. Lead amalgam seals are effective at room temperature but develop leaks at elevated temperatures and also cannot be temperature cycled.